The present invention relates to fasteners. More particularly, the present invention relates to female threaded nuts having a head which can be frangibly removed.
It is vitally important for nut and bolt type fasteners to be torqued to a sufficient value to secure fastening. This is particularly important in the manufacturing of aircraft where lives are at stake. To affix structures together, a bolt is passed through holes formed to align in adjacent structures and a nut is threadably affixed to the bolt. Often, the nut is torqued using a power tool.
To assure that the nut is securely fastened to the bolt, but to avoid over-torqueing the nut, many power driven tools include a clutch mechanism so that the tool will slip so as to not rotate the nut after a predetermined torque has been reached. Unfortunately, power tools may not accurately determine the torque not to exceed or the power tool clutch may inadvertently fail. Consequently, nuts may be inadequately torqued or over-torqued so as to damage the structure or the fasteners.
These problems have been overcome in the past by the use of breakaway nuts wherein the fastener includes a driving head portion. The nut portion and head portion are connected by a thin walled section which is constructed to shear at a predetermined torque. The construction permits the head portion to automatically shear, in other words break off, when a predetermined torque is achieved by a tool, such as a power tool. Accordingly, breakaway nuts do not require that the power tool include a clutch mechanism. Moreover, the breakaway nut construction provides the ability to visually inspect the nut to determine whether it has been adequately torqued. Simply, if the head portion has not twisted off the nut, it is immediately evident that the nut has not been sufficiently torqued.
Unfortunately, previous breakaway nuts suffer from disadvantages. Specifically, breakaway nuts typically have an all-metal construction. The metal provides high strength, but metal fasteners are excessively heavy. Where weight is a premium, such as within aircraft construction, all-metal fasteners pose significant problems. Moreover, metal fasteners can be prone to corrosion including galvanic corrosion.
Attempts have been made to provide fasteners made of plastic or other composite materials so as to reduce weight or corrosion. Unfortunately, the reduction in weight has often caused a corresponding reduction in strength and the ability of the structure to withstand the various stresses to which the assembly may be subjected. For example, U.S. Pat. No. 5,083,888 describes a breakaway nut constructed of a carbon reinforced composite material. The carbon fibers extend perpendicular to the axis of the nut so as to enable the head portion to shear away. Though this structure is not susceptible to corrosion, the construction is relatively expensive to manufacture. Meanwhile, U.S. Pat. No. 8,662,805 describes a breakaway nut which attempts to overcome the problems of galvanic corrosion. The nut portion and head portion are made of metal, but the frangible neck portion is made of a metal that is different than that of the nut and has a higher resistance to corrosion than the base nut material. The frangible section is preferably made of stainless steel so as to provide greater corrosion resistance at the surface where the fastener shears. Unfortunately, this breakaway nut is an all-metal construction which does not provide any weight savings and is expensive to manufacture.
Thus, it would be advantageous to provide an improved frangible nut fastener which possesses high strength.
It would also be advantageous to provide an improved frangible nut fastener which was light weight, inexpensive to manufacture, and particularly acceptable for use within aircraft construction.